Spray combustion phenomena

Abstract

Aspects of interphase transport and multiphase flow relevant to spray combustion are reviewed, considering the structure of the near-injector (dense-spray) region, drop breakup, drop/turbulence interactions and interphase drop transport. Existing measurements of dense-spray structure shown that the dispersedflow flow region is surprisingly dilute, that effects of separated flow are important, that atomization involves primary breakup at the liquid surface followed by secondary breakup, and that effects of drop collisions are small (except when sprays impinge on surfaces or on other sprays). Available information about primary breakup is difficult to interpret because of problems controlling effects of disturbances in injector passages and secondary breakup; therefore, although some understanding of turbulent primary breakup has been achieved, more information about aerodynamic primary breakup is needed to address practical spray combustion processes. Studies of secondary breakup have resolved aspects of breakup regimes and outcomes, including the temporal properties of secondary breakup in some instances, for shock-wave disturbances at small gas/liquid density ratios and liquid viscosities; thus, more information is needed about general drop disturbances and the large gas/liquid density ratio and liquid viscosity condition relevant to spray combustion at high pressures. Several ways to treat the turbulent dispersion of drops within dilute sprays are available. Consequently, the main uncertainties about estimating turbulent dispersion of drops involvelong-standing problems of accurately estimating continuous-phase turbulence properties in sprays. Phenomena that complicate estimates of turbulence properties in sprays include turbulence generation and modulation by drops, with turbulence generation being a dominant but retely studied reature of dense sprays. Understanding of drop evaporation and combustion has increased based on results from numerical simulations allowing for variable properties, multicomponent transport, and detailed chemical kinetic mechanisms; these methods offer promising new ways to study drop ignition, extinction, transport, and behavior near the thermodynamic critical point, among others.